Docs fix : fix a broken link to download js from unpkg cdn

[mznet]

Docs fix:

In Mobx's Installation document, the link to download a js file from unpkg is broken.

The link is currently heading to mobx.umd.js and needed to fix a correct path. But, I can't find a proper path of mobx.umd.js and changed to mobx.umd.production.min.js instead.

[changeset-bot]

⚠️ No Changeset found

Latest commit: ad5af49

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR